Potential measures to reduce nitrate and nitrous oxide losses from renovated grasslands

Samenvatting

Grassland renovation (grassland renewal or temporary conversion to arable land) is a common practice employed by farmers to reverse declining yields in agricultural grasslands. Renovation practices may lead to substantial nitrate (NO₃) leaching and nitrous oxide (N₂O) emissions. Farmers lean towards renovating grasslands in autumn instead of spring because of better sward establishment and low weed infestation. Limited nitrogen (N) uptake of renewed grassland during autumn may, however, increase the potential risk of NO₃ leaching and N₂O emission. Potential options to mitigate these N losses are reduction in tillage intensity and N application rate, or the application of nitrification inhibitors. We assessed the effects of these measures as well as timing of renewal and the conversion to grassland to maize on NO₃ leaching and N₂O emissions after one growing season. Five replicated field experiments were conducted of one year each at five locations in the Netherlands. We measured crop yield, soil mineral N, NO₃ concentrations in groundwater and N₂O emission. Averaged over all experiments, the measures had an effect on NO₃ concentration in groundwater, although not significant. The highest NO₃ concentrations in groundwater were observed after autumn renewal (17.2 mg NO₃-N L⁻¹), autumn renewal combined with mitigation strategies (12.8–19.4 mg NO₃-N L⁻¹) and conversion to maize (12.5–12.8 mg NO₃-N L⁻¹), with all of these treatments exceeding the European norm of 11.3 mg NO₃-N L⁻¹. Concentrations after renewal in spring were lower (3.1 mg NO₃-N L⁻¹) and did not lead to an increase compared to the control (4.4 mg NO₃-N L⁻¹). N₂O emissions increased significantly directly after grassland renewal relative to the reference grassland and N₂O fluxes were similar in spring and autumn. We conclude that autumn renewal leads to significant increases in the risk of NO₃ leaching which cannot be mitigated by the technical measures in this study.